You can get an iPhone on Virgin Mobile for $35-55 per month depending on your voice minute needs (probably high). Data is unlimited, but you get what you pay for. Virgin Mobile uses Sprint as their back-end and the 3G speeds are atrocious almost anywhere you go.

If it doesn't have to be an iPhone, you can get a Samsung Galaxy Proclaim for $150 at Walmart. It's $45 per month for the Straight Talk service. For this phone, you're using the Verizon network. However, Straight Talk is also an MVNO for T-Mobile, AT&T and Sprint. Be sure to check the map on the side of the box.

For what it's worth, you can also get an iPhone 5 on Straight Talk's Verizon service. Same $45 per month. You're going to pay for the phone upfront, though.